1. Field of the Invention
The present invention relates to a secondary battery and, more particularly, to a case of a secondary battery having an improved structure at a portion for sealing a battery unit.
2. Description of the Related Art
Lithium secondary batteries are classified into liquid electrolyte batteries and polymer electrolyte batteries according to the type of electrolyte used. Generally, batteries using liquid electrolyte are called lithium ion batteries and batteries using polymer electrolyte are called lithium polymer batteries.
Among these batteries, lithium polymer batteries use a polymer electrolyte so that they are free of danger of explosion. Also, the lithium polymer batteries have solved the shortcoming of lowering of the battery performance due to byproducts produced during charging or discharging. Further, the lithium polymer batteries are relatively flexible in design shape.
FIGS. 1A and 1B illustrate an example of a conventional lithium polymer battery 10.
As shown in FIG. 1A, the conventional lithium polymer battery 10 includes a battery unit 14 having a plurality of assembled batteries, each battery having a positive electrode 11, a negative electrode 12, and a separator 13 interposed therebetween and laminated. A positive electrode tab 15 is drawn out from one side periphery of the positive electrode 11 and a negative electrode tab 16 is drawn out from one side periphery of the negative electrode 12. The positive and negative electrode tabs 15 and 16 are welded to positive and negative electrode terminals 17 and 18, respectively.
The aforementioned battery unit 14 is mounted in a case 19 having a space in which the battery unit 14 is accommodated. The case 19 includes upper and lower case bodies 19a and 19b which are capable of being separated.
Here, the case 19 has a multi-layered structure.
As shown in FIG. 1B, the case 19 has a structure in which a first poly(ethylene-co-acrylic acid) (EAA) layer 110, a polyethylene (PE) layer 120, a first nylon layer 130, a second EAA layer 140, an aluminum film 150, and a second nylon layer 160 are sequentially laminated.
Here, the above-stated multiple layers function differently. In other words, the first EAA layer 110 and the second EAA layer 140 prevents a swelling effect of an electrolytic solution so that the electrolytic solution does not stick to the aluminum film 150 during the operation of the battery 10. The PE layer 120 enhances the formability of the case 19. The aluminum foil layer 150 prevents moisture from infiltrating into the battery 19 and suppresses the loss of the electrolytic solution. Also, the first and second nylon layers 130 and 160 prevent cracks from being generated in the case 19 and enhance formability.
However, in the case 19 having the aforementioned configuration, since the first and second EAA layers 110 and 140 or the PE layer 120 are not completely insoluble in a small amount of the electrolytic solution injected into the case 19, they may react with the battery 10 during the operation or storage thereof, causing contamination. Also, since the case 19 is weakened by heat, deformation of the battery 10 may be caused due to a raised internal temperature during the operation of the battery 10, deteriorating the battery performance.
To solve the above problems, it is an object of the present invention to provide a case of a secondary battery having an improved structure for improving the battery performance.
Accordingly, to achieve the above object, there is provided a case of a secondary battery, in which a battery unit having a positive electrode, a separator, and a negative electrode laminated in sequence, is accommodated to then be sealed, the case including a fluorocarbon resin based polymer layer as the innermost layer of the case, a nylon layer as the outermost layer of the case, and an aluminum foil layer between the fluorocarbon resin based polymer layer and the nylon layer.
Preferably, the fluorocarbon resin based polymer layer is a polytetrafluoroethylene (PTFE) layer. Here, the PTFE layer is a single layer of an aqueous emulsion type.
Further, the PTFE layer is preferably a double layer consisting of a first PTFE layer of an aqueous emulsion type, formed as the innermost layer of the case, and a second PTFE layer formed between the first PTFE layer and the aluminum foil layer, for improving adhesion by adding acid and a plasticizer.
A polyethylene terephthalate (PET) layer may be further formed on the nylon layer.